An a-MAIZE-ing Geneticist

Associate Professor Paula McSteen is the recipient of the 2017 Mid-Career Excellence in Maize Genetics Award.

Paula McSteen may be dwarfed by the corn plants she studies, but her scientific stature in the maize genetics community continues to grow.

McSteen, an associate professor in the Division of Biological Sciences and investigator in the Christopher S. Bond Life Sciences Center, was recently awarded the Mid-Career Excellence in Maize Genetics Award by the Maize Genetics Executive Committee. The award is given annually to a scientist who started a lab in the last 15 years and has produced an outstanding track record of discovery research in the area of maize genetics.

The award was announced on 10 March at a ceremony in conjunction with the 59th Annual Maize Genetics Conference in St. Louis. At the ceremony, the Irish-born biologist shared a limerick that she said conveyed the upside and downside of working with corn.

There was a young lassie named Paula
Worked on corn, she wished she was tall-a’
When she’d reach for a tassel
She’d exclaim, “What a hassle!”
But the mutants she loved most of all-a’

All jokes aside, McSteen said receiving the award was “very meaningful” to her.

“This award is very special to me because it comes from the maize genetics community,” she said. “It is a wonderfully collaborative community, and I could not have done this research without the amazing genetics and genomics resources generated by the community.”

McSteen was recognized for her research on the role the plant hormone auxin plays in the development of organs in corn. Unlike humans, plants can continually generate new organs. This process is mediated by special cells, called meristems, which are analogous to stems cells in animals. Axillary meristems are produced along the main stem where the buds or shoots develop. In corn, the axillary meristem cells give rise to the tassels and ears, the most crucial organs for plant yield. Which genes control initiation of the axillary meristems in the tassel and ear and how this process is governed by auxin have been the fundamental questions guiding her work.

Fellow maize geneticist James Birchler, a Curators’ Distinguished Professor of biological sciences, said McSteen is “richly deserving” of the recognition.

“Paula’s research has contributed much to our current understanding of auxin in maize. Her laboratory cloned genes involved in auxin biosynthesis and transport and provided the first genetic evidence that auxin is synthesized in a two-step process from a common amino acid. More recently, her lab demonstrated that boron is necessary for meristem maintenance and identified a genetic mechanism for the development of a new meristem type,” he said.

He added that McSteen also has provided “considerable service and valuable leadership” to the broader maize genetics community.

McSteen with members of her lab at the Maize Genetics Conference.

Cracking the Auxin Code

Plant hormones, like human hormones, are small chemical signaling molecules that tell cells to turn specific genes on or off. The hormone auxin affects nearly all aspects of plant growth and development, from stem growth and flower formation to root patterning and stress response.

“Auxin is a really tiny molecule, but yet it has amazingly diverse functions in different parts of the plant and in different plants. How can this one simple molecule give rise to such a diverse array of responses? This has been one of those big and long-standing questions of plant biology,” McSteen said.

Previous studies have shown that there are few chemical steps between when a cell perceives auxin and when a gene is expressed. The chemical pathway involves three molecular players – a receptor, a repressor, and a transcription factor – that, upon perception of auxin, act in a coordinated manner on target genes. On the face of it, this simple mechanism seems to add to the conundrum. Not so, according to McSteen.

“It seems simple until you recognize that there are 8 versions of the receptor, 33 versions of the repressor, and 34 versions of the transcription factor,” she said. “These multiple versions of each component of the pathway allows for a lot of possible combinations. That’s why we call it a network.”

A network conjures up an image of a complex web of distinct but interconnecting and interrelated lines and nodes. The term is used to describe everything from social relationships to linked computer systems to neurons in the brain. Ultimately, the term implies interactions. As it relates to auxin, the network derives from the fact that a single receptor can interact with any number of the 33 repressors, which, in turn, can interact with any number of the 34 transcription factors to turn on hundreds of target genes.

McSteen hypothesizes that the possible combinations explain how this one simple molecule can be involved in such an immense range of biological processes. She was recently awarded a five-year, $3.6 million research grant from the National Science Foundation to lead a team of researchers on a project designed to “crack the auxin code” that gives rises to the ears and tassels in corn.

To do this, McSteen and her collaborators are using genetic and molecular approaches to identify the specific versions of the auxin components functioning in the tassels and ears, identify target genes, and then use bioinformatics techniques to visualize the network of all possible combinations. The researchers then will introduce the different auxin-response components into modified yeast cells to measure how each regulates gene expression in the presence of auxin. Together, the experiments will reveal the “code” of auxin signaling.

Corn is a major crop in the United States, and its reproductive organs directly contribute to yield. McSteen expects the results of the project will be directly translatable from the lab to the field. She also anticipates that the tools and approaches will empower other researchers to decipher the auxin code in other biological processes and in other plants.